When drilling a well (e.g., for oil or gas), a drill bit is attached to the end of a drill string and drills a hole through rock to access the oil or gas reservoir. Drilling mud is used during drilling operations. Drilling fluid comprises, for example, a finely ground clay base material to which various chemicals and water are added to form a viscous fluid. This drilling mud is pumped down the hollow drill pipe, through the drill bit and returned to the surface in the annular space between the drill pipe and the well bore.
The drilling mud serves three main purposes. First, it aids in cooling the drill bit and thereby increasing its useful life. Second, the mud flushes the cuttings or “solids” from the well bore and returns them to the surface for processing by a solid control system. Third, the mud leaves a thin layer of the finely ground clay base material along the well bore walls which helps prevents caving in of the well bore wall.
Although often referred to simply as “mud”, the drilling mud is a complex composition which must be carefully engineered and tailored to each individual well and drilling operation. Thus the drilling fluid is costly and thus is cleaned and reused in a closed loop system in which a solid control system and a shaker play important roles.
A shaker, often referred to as a “shale shaker,” is part of a solids control system used in oil and gas drilling operations to separate the solid material (“solids”), removed from the well bore by the drilling operation, from the drilling mud. In order for the drilling fluid to be used and reused it must be processed after returning from the well bore to remove the aforementioned solids and maintain its proper density, often expressed as pounds per gallon or “mud weight”, i.e., 10 lb./gal. mud or “10 lb. mud”. The first step in processing the returned drilling fluid is to pass it through a shaker with a screen. The vibrating action of the screen over which the mud passes removes larger particle size solids (e.g., in the 200 to 700 micron size range) while allowing the drilling fluid and smaller particle size solids to pass through the screen.
Solids, which are discarded from the top of the shaker screen, discharge into a pit or onto a conveyor and the underflow drilling fluid flows into the tank below. The drilling fluid in the tank (which still includes smaller size particles in the mud) is stirred with an agitator before being pumped to additional solids control equipment known as desanders and desilters. The desander removes abrasive solid particles smaller in size than what the shaker screens remove. The desilter then removes solids at even smaller sizes still. At this point the underflow from the desander and desilter, which is mostly solids with a small amount of drilling fluid is then sent to a third shaker referred to as a drying shaker or mud cleaner. This shaker has a very fine screen to allow the drying of the removed solids and recapture of as much of the costly drilling fluid as possible. The drilling fluid is then processed through a centrifuge to remove solids down to very small sizes (e.g., 2 microns) before being recirculated into the well bore.
The present disclosure relates to the shale shaker. A shaker includes a support frame of some sort on which a vibrating assembly is driven with a motor. The vibrating motion of the vibrating assembly causes the support frame and thus the screens to vibrate.
One type of shaker is known as a “circular motion” shaker in which the excitation force comes from the centrifugal force generated by a rotating eccentric weight driven by a single motor. Circular motion shakers generally require the screen deck to be angled downward in the desired direction of mud flow in order to achieve sufficient mud flow across the screens. This downward angle unfortunately limits the screen mesh size to coarser screen meshes in order to maintain a minimal flow rate. Too fine a mesh and the mud will simply flow downhill and off the discharge end, before the affluent has a chance to conduct through the fine mesh screen. Such shakers generally require lower capital cost and achieve higher screen life due to lower peak G-forces. However, such shakers are limited to coarser screen meshes, have limited or no deck angle adjustment due to required downward sloping deck angle, have relatively poor solids conveyance, have lower peak G-force, and experience generally sub-standard overall performance.
A linear motion shaker includes two identical vibrator motors driven at the same speed but the eccentric weights associated with each motor rotate in opposite directions. This results in more linear excitation profile and thus maximum G-force is generated generally along a single axis. Because of the linear nature of the vibration profile, a linear motion shaker can “aim” the axis of excitation by adjusting the angle of the vibratory motor assemblies thereby achieving desired solids conveyance characteristics regardless of deck angle. The deck angle then can be adjusted to speed up or slow down solids conveyance depending on the prevailing conditions. Further, a wider range of screen mesh sizes can be used and the deck angle can be adjusted to “tune” for the desired balance between optimum solids conveyance and maximum flow rate handling. Relatively high G-forces achievable by linear motion shakers unfortunately result in shorter screen life.
Balanced elliptical motion shakers detune the sharp focus of the linear shaker to produce a softer and gentler ovoid motion. As long as the width of the ovoid motion is not stretched so far as to approximate circular motion, balanced elliptical motion shakers can retain most of the performance characteristics of linear motion shakers, while extending screen life. The added lateral motion may also help clear the screen surface of pluggage caused by sticky reactive clay. Balanced elliptical shakers result in reduced peak G-forces, increased solids contact time with the screens and decreased solids conveyance efficiency. Balanced elliptical shakers have relatively high capital cost.
FIG. 1 illustrates the relative motions of linear, balanced elliptical, and circular motion shakers.
The shakers described above are all driven by vibrator motors that comprise an eccentric mass that rotates at a constant distance from the center of rotation. The mass also rotates at a constant angular velocity matched to the rotational speed of the motor. Further, the motion is balanced about both the axis of rotation and the plane normal to the axis of rotation. All such shakers lack true “lateral motion,” each having only varying widths in their elliptical orbits. None of these shakers simultaneously optimizes all of the performance characteristics including peak G-force, solids conveyance, screen contact time, screen self-cleaning and screen life. For example, increasing the eccentric weight in a linear motion shaker increases peak G-force and solids conveyance but decreases screen contact time and screen life. Increasing the elliptical path of elliptical motion shakers increase screen contact time and screen life but at the detriment of peak G-force and solids conveyance.